I need to adjust the valves on my briggs ohv motor. How do I set the piston 1/4" past top dead center ?

First you need to determine the rotational direction of the engine. Then you need to determine if, at top dead center it is on compression or exhaust by watching the valves. Once you have determined the TDC on compression, insert a thin ruler into the cylinder and sit it on the piston. Note the reading of the ruler at the spark plug hole lip. Add 1/4" to that marking and rotate the engine until it is at that mark. For example, if at TDC the ruler reads 3/8 ", you add 1/4" =5/8 inch. Rotate the engine until 5/8 " is at the plug hole lip. Now you are at 1/4" past TDC and can adjust the valves.

Witch valve is intake /exh. On 14.5 briggs

Hi,
I realize that the manual says that, but try doing the way shown in the YouTube video:
Lawn Mower Repair Valve Adjustment - YouTube (http://www.youtube.com/watch?v=NjzX-PUjKmo)
Peace,
Clarke